1. Technical Field
The present invention generally relates to methods for determining the effectiveness of asphaltene dispersant additives for inhibiting or preventing asphaltene precipitation in a hydrocarbon-containing material subjected to elevated temperature and pressure conditions.
2. Description of the Related Art
Hydrocarbon-containing feedstocks generally contain polar core materials, such as asphaltenes, dispersed in lower polarity solvent(s). Intermediate polarity material(s), usually referred to as resin(s), can associate with the polar core materials to maintain a homogeneous mixture of the components.
The large reserves of heavy or extra heavy crude oil are very viscous in their natural state. The viscous nature of the crude oil, however, makes it difficult to transport the oil in conventional pipelines to stations where it can be processed into useful end products. The origin of high viscosity in these oils has been attributed to high asphaltene content of the oils. Asphaltenes are organic heterocyclic macromolecules which occur in crude oils. Under normal reservoir conditions, asphaltenes are usually stabilized in the crude oil by maltenes and resins that are chemically compatible with asphaltenes, but that have lower molecular weight. Polar regions of the maltenes and resins surround the asphaltene while non-polar regions are attracted to the oil phase. Thus, these molecules act as surfactants and result in stabilizing the asphaltenes in the crude. However, changes in pressure, temperature or concentration of the crude oils can alter the stability of the dispersion and increase the tendency of the asphaltenes to agglomerate into larger particles. As these asphaltene agglomerates grow, so does their tendency to precipitate out of solution. Generally, unwanted asphaltene precipitation is a concern in upstream applications due to, for example, plugging of an oil well or pipeline as well as stopping or decreasing oil production.
In downstream applications, asphaltenes are believed to be the source of coke during thermal upgrading processes thereby reducing and limiting yield of residue conversion. Presently, the petroleum industry relies more heavily on relatively high boiling feedstocks derived from materials such as coal, tar sands, oil-shale, and heavy crudes. These feedstocks generally contain significantly more undesirable components, especially from an environmental point of view. Consequently, such feedstocks and product streams require more severe upgrading in order to reduce the content of such undesirable components. More severe upgrading, of course, adds considerably to the expense of processing these petroleum streams.
As discussed above, asphaltenes are believed to be the source of coke during thermal upgrading processes thereby reducing and limiting yield of residue conversion. In catalytic upgrading processes, asphaltenes can contribute to catalyst poisoning by coke and metal deposition thereby limiting the activity of the catalyst. Asphaltenes can also cause fouling in, for example, heat exchangers and other equipment in a refinery. Fouling in heat transfer equipments used for streams of petroleum origin can result from a number of mechanisms including chemical reactions, corrosion and the deposit of materials made insoluble by the temperature difference between the fluid and heat exchange wall. The presence of insoluble contaminants may exacerbate the problem: blends of a low-sulfur, low asphaltene (LSLA) crude oil and a high-sulfur, high asphaltene (HSHA) crude, for example, may be subject to a significant increase in fouling in the presence of iron oxide (rust) particulates. Subsequent exposure of the precipitated asphaltenes over time to the high temperatures then causes formation of coke as a result of thermal degradation.
The precipitation of asphaltenes can be prevented or reduced by small amounts of dispersants. These dispersants display one or more of the following effects:
a) the amount of precipitate is reduced;
b) the precipitate is formed more slowly;
c) the precipitate is more finely divided; and
d) the tendency of the precipitate to deposit on surfaces is reduced.
Accordingly, it would be advantageous to characterize a hydrocarbon-containing material such as a hydrocarbon-containing feedstock which has been subjected to elevated temperature and pressure conditions, i.e., reservoir or process conditions, in order to minimize problems associated with asphaltene precipitation in upstream and downstream applications. Thus, it would be desirable to provide improved methods for determining the effectiveness of one or more asphaltene dispersant additives for inhibiting or preventing asphaltene precipitation in a hydrocarbon-containing material subjected to elevated temperature and pressure conditions.